Image forming apparatus including exposure head provided with plurality of light emitting chips

ABSTRACT

An image forming apparatus includes a light emitting chip and at least one processor. The light emitting chip includes a plurality of light emitting elements, a DAC outputting a voltage corresponding to a setting value, and a circuit unit that supplies a current to the plurality of light emitting elements based on the voltage. At least one processor is configured to set the setting value such that one light emitting element included among the plurality of light emitting elements emits light of a predetermined amount, and correct image data pieces that respectively correspond to the plurality of light emitting elements based on first correction data for correcting amounts of light respectively emitted by the plurality of light emitting elements. The circuit unit supplies a current to each of the plurality of light emitting elements based on the corrected image data pieces.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an image forming apparatus of anelectrophotographic method.

Description of the Related Art

An image forming apparatus of an electrophotographic method forms anelectrostatic latent image on a photosensitive member, which is drivento rotate, by exposing the photosensitive member to light, and forms animage by developing this electrostatic latent image using toner. Notethat the direction parallel to a rotation axis of the photosensitivemember is referenced as a main scanning direction. Japanese PatentLaid-Open No. 2018-1679 discloses an image forming apparatus in which aplurality of chips including a plurality of light emitting elements arearrayed in the main scanning direction, and which exposes one line inthe main scanning direction to light. Japanese Patent Laid-Open No.2018-1679 discloses a configuration that corrects density unevennesscaused by a difference between light amounts of two chips that neighboreach other in the main scanning direction.

However, the difference between light amounts can arise not only betweenchips, but also among a plurality of light emitting elements inside achip. Therefore, correcting only the difference between light amounts ofchips can still leave the possibility that density unevenness appears ina formed image. That is to say, density unevenness appears in an imageformed by the configuration of Japanese Patent Laid-Open No. 2018-1679.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, an image formingapparatus, includes: a photosensitive member that is driven to rotate;an exposure head including a first light emitting chip and a secondlight emitting chip that is placed at a position different from aposition of the first light emitting chip in a direction along arotation axis of the photosensitive member, the first light emittingchip including a plurality of first light emitting elements that areplaced at different positions in the direction along the rotation axisof the photosensitive member, a first digital-analog converter thatoutputs a voltage corresponding to a setting value as a digital value,and a first circuit unit that supplies a current to the plurality offirst light emitting elements based on the voltage output from the firstdigital-analog converter, the second light emitting chip including aplurality of second light emitting elements that are placed at differentpositions in the direction along the rotation axis of the photosensitivemember, a second digital-analog converter that outputs a voltagecorresponding to a setting value as the digital value, and a secondcircuit unit that supplies a current to the plurality of second lightemitting elements based on the voltage output from the seconddigital-analog converter; and at least one processor configured to set avalue that has been decided on so that one light emitting elementincluded among the plurality of first light emitting elements emitslight of a predetermined amount as the setting value in the first lightemitting chip, and set a value that has been decided on so that onelight emitting element included among the plurality of second lightemitting elements emits light of the predetermined amount as the settingvalue in the second light emitting chip, generate image data pieces forcausing the plurality of light emitting elements to emit light, andcorrect image data pieces that respectively correspond to the pluralityof first light emitting elements based on first correction data forcorrecting amounts of light respectively emitted by the plurality offirst light emitting elements, and correct image data pieces thatrespectively correspond to the plurality of second light emittingelements based on second correction data for correcting amounts of lightrespectively emitted by the plurality of second light emitting elements,wherein the first circuit unit supplies a current to each of theplurality of first light emitting elements based on the corrected imagedata pieces, and the second circuit unit supplies a current to each ofthe plurality of second light emitting elements based on the correctedimage data pieces.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram of an image forming apparatusaccording to an embodiment.

FIG. 2A and FIG. 2B are diagrams showing an exposure head and aphotosensitive member according to an embodiment.

FIG. 3A and FIG. 3B are diagrams showing a printed circuit board of theexposure head according to an embodiment.

FIG. 4 is a diagram illustrating the arrangement of light emittingelements inside light emitting chips according to an embodiment.

FIG. 5 is a plan view of a light emitting chip according to anembodiment.

FIG. 6 is a cross-sectional view of a light emitting chip according toan embodiment.

FIG. 7 is a diagram showing spots on a photosensitive member accordingto an embodiment.

FIG. 8 is a diagram of a configuration for controlling each lightemitting chip according to an embodiment.

FIG. 9 is a diagram of blocks inside a light emitting chip according toan embodiment.

FIG. 10 is a block diagram of a light amount correction unit accordingto an embodiment.

FIG. 11A to FIG. 11D are diagrams illustrating processing in the lightamount correction unit according to an embodiment.

FIG. 12 is a diagram showing a light amount correction chart accordingto an embodiment.

FIG. 13 is a diagram illustrating processing for generating correctioninformation according to an embodiment.

FIG. 14 is a diagram illustrating processing for generating correctioninformation according to an embodiment.

FIG. 15A and FIG. 15B are diagrams illustrating processing forgenerating correction information according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments will be described in detail with reference tothe attached drawings. Note, the following embodiments are not intendedto limit the scope of the claimed invention. Multiple features aredescribed in the embodiments, but limitation is not made to an inventionthat requires all such features, and multiple such features may becombined as appropriate. Furthermore, in the attached drawings, the samereference numerals are given to the same or similar configurations, andredundant description thereof is omitted.

FIG. 1 is a schematic configuration diagram of an image formingapparatus according to the present embodiment. A reading unit 100optically reads an original placed on a platen, and generates image dataindicating the result of reading. An image creation unit 103 forms animage on a sheet, for example, based on the image data generated by thereading unit 100, or based on image data received from an externalapparatus via a network.

The image creation unit 103 includes image forming units 101 a, 110 b,101 c, and 101 d. The image forming units 101 a, 101 b, 101 c, and 101 dform black, yellow, magenta, and cyan toner images, respectively. Theconfigurations of the image forming units 101 a, 101 b, 101 c, and 101 dare similar to one another; hereinafter, they are also collectivelyreferred to as image forming units 101. At the time of image formation,a photosensitive member 102 of an image forming unit 101 is rotated in aclockwise direction of the figure. A charger 107 charges thephotosensitive member 102. An exposure head 106 exposes thephotosensitive member 102 to light in accordance with image data, andforms an electrostatic latent image on the photosensitive member 102. Adeveloper 108 develops the electrostatic latent image on thephotosensitive member 102 using toner. The toner image on thephotosensitive member 102 is transferred to a sheet conveyed on atransfer belt 111. Note that colors different from black, yellow,magenta, and cyan can be reproduced by transferring the toner images onthe respective photosensitive members 102 in such a manner that thetoner images overlap one another.

A conveyance unit 105 controls feeding and conveyance of sheets.Specifically, the conveyance unit 105 feeds a sheet to a conveyance pathin the image forming apparatus from a designated unit among internalstorage units 109 a and 109 b, an external storage unit 109 c, and amanual feed unit 109 d. The sheet that has been fed is conveyed to aregistration roller 110. The registration roller 110 conveys the sheetonto the transfer belt 111 at a predetermined timing so that the tonerimages on the respective photosensitive members 102 are transferred tothe sheet. As stated earlier, the toner images are transferred to thesheet while the sheet is conveyed on the transfer belt 111. A fixingunit 104 applies heat and pressure to the sheet to which the tonerimages have been transferred, thereby fixing the toner images on thesheet. After the toner images have been fixed, the sheet is dischargedto the outside of the image forming apparatus by a discharge roller 112.Note that an optical sensor 113 is placed in a position facing thetransfer belt 111. The optical sensor 113 detects a test chart formeasuring an amount of color misregistration, which is formed on thetransfer belt 111 by the image forming unit 101. A control unit, notshown in the figure, performs color misregistration correction controlbased on the detection result of the test chart.

FIG. 2A and FIG. 2B show the photosensitive member 102 and the exposurehead 106. The exposure head 106 includes a group of light emittingelements 201, a printed circuit board 202 on which the group of lightemitting elements 201 is mounted, a cylindrical lens array 203, and ahousing 204 for attaching the cylindrical lens array 203 to the printedcircuit board 202. The cylindrical lens array 203 forms a formed-imagespot (hereinafter simply referred to as a spot) of a predetermined sizeon the photosensitive member 102 by collecting light emitted by thegroup of light emitting elements 201 on the photosensitive member 102.

FIG. 3A and FIG. 3B show the printed circuit board 202. Note that FIG.3A shows a surface on which a connector 305 is mounted, and FIG. 3Bshows a surface on which the group of light emitting elements 201 ismounted (a surface opposite to the surface on which the connector 305 ismounted). In the present embodiment, the group of light emittingelements 201 includes 20 light emitting chips 400-1 to 400-20. The lightemitting chips 400-1 to 400-20 are arrayed in a two-row zigzag patternalong the main scanning direction. More specifically, light emittingchips 400-(2 k−1) (where k is an integer from 1 to 10) are arranged in arow along the main scanning direction, and light emitting chips 400-2 kare arranged in a row along the main scanning direction. The position ofthe row of light emitting chips 400-(2 k−1) in the sub scanningdirection is different from the position of the row of light emittingchips 400-2 k in the sub scanning direction. Note that the sub scanningdirection is the direction corresponding to the direction of rotation ofthe photosensitive member 102. Also, the sub scanning direction is thedirection perpendicular to the main scanning direction. In the followingdescription, the light emitting chips 400-1 to 400-20 are alsocollectively referred to as light emitting chips 400. Furthermore, thelight emitting chips 400-(2 k−1) are referred to as light emitting chips400 of an odd-number row, and the light emitting chips 400-2 k arereferred to as light emitting chips 400 of an even-number row. Eachlight emitting chip 400 includes a plurality of light emitting elements.Each light emitting chip 400 on the printed circuit board 202 isconnected to an image controller 800 (FIG. 8 ), which is a control unit,via the connector 305.

FIG. 4 is a diagram illustrating the arrangement of the light emittingchips 400. In a light emitting chip 400, four sets of light emittingelements 602 are arrayed in the sub scanning direction, each setincluding 748 light emitting elements arrayed along the main scanningdirection. The pitch of light emitting elements 602 that neighbor eachother in the main scanning direction is approximately 21.16 μm, whichcorresponds to a resolution of 1200 dpi. Therefore, the length of 748light emitting elements in one set in the main scanning direction isapproximately 15.8 mm. Note that the sets are placed in such a mannerthat they are shifted from one another in the main scanning direction byapproximately 5 μm, which corresponds to a resolution of 4800 dpi.Furthermore, a light emitting chip 400 in the even-number row and alight emitting chip 400 in the odd-number row are placed so that theyoverlap in the main scanning direction. An interval Ly between the lightemitting elements 602 in a light emitting chip 400 in the even-numberrow and the light emitting elements 602 in a light emitting chip 400 inthe odd-number row is, for example, approximately 105 μm.

FIG. 5 is a plan view of a light emitting chip 400. A light emittingchip 400 has a light emitting unit 404 that includes a plurality oflight emitting elements 602. The light emitting unit 404 is formed on alight emitting substrate 402. Also, a circuit unit 406 for controllingthe light emitting unit 404 is provided on the light emitting substrate402. Aline for communication with an image controller 800 is connectedto pads 408.

FIG. 6 shows a part of a cross-section taken along the line A-A of FIG.5 . A plurality of lower electrodes 504 are formed on the light emittingsubstrate 402. A gap having a length dx is present between twoneighboring lower electrodes 504. Alight emitting layer 506 is providedon the lower electrodes 504, and an upper electrode 508 is provided onthe light emitting layer 506. The upper electrode 508 is one sharedelectrode that corresponds to the plurality of lower electrodes 504.When a predetermined voltage is applied between the lower electrodes 504and the upper electrode 508, a current flows from the lower electrodes504 to the upper electrode 508, thereby causing the light emitting layer506 to emit light. That is to say, a lower electrode 504 is provided incorrespondence with one light emitting element 602. By making the lengthdx large relative to the length dz between the lower electrodes 504 andthe upper electrode 508, a leakage current between neighboring lowerelectrodes 504 can be suppressed, and erroneous light emission byneighboring light emitting elements 602 can be suppressed.

For example, an organic EL film can be used as the light emitting layer506. Furthermore, an inorganic EL film can be used as the light emittinglayer 506. The upper electrode 508 is composed of, for example, atransparent electrode, such as indium tin oxide (ITO), so as to allowthe light emission wavelength of the light emitting layer 506 to betransmitted therethrough. Note that although the entirety of the upperelectrode 508 allows the light emission wavelength of the light emittinglayer 506 to be transmitted therethrough in the present embodiment, itis not necessary for the entirety of the upper electrode 508 to allowthe light emission wavelength to be transmitted therethrough.Specifically, it is sufficient that the light emission wavelength betransmitted through the regions via which light beams from therespective light emitting elements 602 (corresponding to the lowerelectrodes 504) are emitted.

As has been described using FIG. 4 , one light emitting chip 400includes four sets of multiple light emitting elements 602 that arearranged along the main scanning direction, and one of the sets thatneighbor each other in the sub scanning direction is shifted from theother in the main scanning direction by 5 μm. In exposing one line ofthe photosensitive member 102 to light, the light emission timings ofthe four sets are controlled so as to expose this line of thephotosensitive member 102 to light. Therefore, as shown in FIG. 7 , thefour light emitting elements 602 in the four sets that are located atthe substantially same position in the main scanning direction exposethe photosensitive member 102 to light at the positions that are shiftedfrom one another by 5 μm. In this way, as the spots made by therespective light emitting elements 602 overlap one another, a smoothelectrostatic latent image is formed. Note that although the number ofsets is four in the present embodiment, the number of sets can be two ormore.

As described above, the exposure head 106 according to the presentembodiment includes 20 light emitting chips 400 that are arrayed in atwo-row zigzag pattern along the main scanning direction, and each lightemitting chip 400 includes four sets of multiple light emitting elements602 that are arrayed along the main scanning direction. The sets arearranged along the sub scanning direction, and the position of one ofneighboring sets in the main scanning direction is shifted from theposition of the other in the main scanning direction by 5 μm, whichcorresponds to a resolution of 4800 dpi. Looking the entirety of theexposure head 106, the respective positions of the plurality of lightemitting elements 602 in the main scanning direction differ from oneanother. Note that although the positions of the plurality of lightemitting elements 602 in the sub scanning direction are not the same,the light emission timings of the respective light emitting elements 602are adjusted so as to expose the same line of the photosensitive member102 to light. Therefore, the spots that are respectively made by theplurality of light emitting elements 602 can be formed on one line,along the main scanning direction, on the photosensitive member 102 atan interval of approximately 5 μm. In the following description, thepositions at which the plurality of light emitting elements 602respectively form the spots are referred to as “dots”. Furthermore, in acase where a light emitting element 602 is caused to emit light at aposition of a dot, this dot is referred to as an “exposure dot”; in acase where a light emitting element 602 is not caused to emit light at aposition of a dot, this dot is referred to as a “non-exposure dot”.

FIG. 8 shows a configuration in which the image controller 800 controlseach light emitting chip 400. Image data indicating the tone values ofrespective pixels in an image to be formed is input to an image datageneration unit 801. The image data generation unit 801 performsdithering processing (halftone processing) with respect to this imagedata in accordance with a resolution designated by a CPU 811, andoutputs the image data after the processing to a light amount correctionunit 802. The image data after the halftone processing indicates whethereach dot that composes the image is to be an exposure dot or anon-exposure dot. In other words, the image data after the halftoneprocessing indicates whether to cause the light emitting elements 602corresponding to the respective dots to emit light. The light amountcorrection unit 802 performs light amount correction with respect to theimage data based on correction information, and outputs the image dataafter the light amount correction to a chip data conversion unit 803. Asynchronization signal generation unit 804 generates a linesynchronization signal (Lsync) 808. The line synchronization signal 808is used to determine, from the image data, a data portion correspondingto one line of the photosensitive member 102 in the main scanningdirection. The chip data conversion unit 803 transmits image data (DATA)807 corresponding to one line to each light emitting chip 400 insynchronization with the line synchronization signal 808. Note that achip selection signal (CS) 805 indicates to which light emitting chip400 the image data 807 is addressed. Furthermore, the chip dataconversion unit 803 transmits a clock signal (CLK) 806 to each lightemitting chip 400.

Correction information, which will be described later, is stored in astorage unit 810 of the printed circuit board 202. Note that each blockshown in FIG. 8 is configured to be capable of exchanging various typesof information by way of transmission and reception of a control signal(CTL) 809. Upon receiving the image data 807 from the chip dataconversion unit 803, each light emitting chip 400 performs a lightemitting operation in accordance with the received image data 807 at aninput timing of the next line synchronization signal 808.

FIG. 9 is a block diagram of a light emitting chip 400. A digital/analogconverter (D/A) 901 outputs an analog voltage corresponding to a digitalvalue, which is a setting value that has been set by the CPU 811. Thisdigital value is indicated by the correction information; the CPU 811determines the digital value to be set for the D/A 901 in each lightemitting chip 400 by reading out the correction information stored inthe storage unit 810. The light emitting elements 602 in a lightemitting chip 400 are grouped into a plurality of blocks in the mainscanning direction. Each group is provided with one correspondingreference current source 902. In FIG. 9 , the light emitting elements602 are grouped into five groups, and thus the light emitting chip 400includes reference current sources 902-1 to 902-5 that respectivelycorrespond to the groups. The reference current sources 902-1 to 902-5output a reference current corresponding to the output value output fromthe D/A 901, that is to say, the analog voltage, to each light emittingelement 602 in the corresponding group. In this way, the D/A 901functions as a current control unit that that controls a referencecurrent to the light emitting elements 602. The light emission amountsof the light emitting elements 602 are controlled based on the referencecurrent output from the corresponding reference current source 902.

FIG. 10 is a block diagram of the light amount correction unit 802. Thecorrection information indicates light amount correction values A, lightamount correction values B, and spot correction values C. The CPU 811notifies the light amount correction unit 802 of the light amountcorrection values A, light amount correction values B, and spotcorrection values C by reading out the correction information stored inthe storage unit 810.

The light amount correction values A are correction values forcorrecting the light amount differences among the groups of a lightemitting chip 400. In the present embodiment, as the light emittingelements 602 inside one light emitting chip 400 are grouped into fivegroups, five light amount correction values A are set with respect toone light emitting chip 400. One group, that is to say, one referencecurrent source 902 is associated with one light amount correction valueA.

The light amount correction values B are correction values forcorrecting the light amount differences among the light emittingelements 602 inside a group. While the details will be described later,four light amount correction values B are associated with one group ofone light emitting chip 400 in the present embodiment. For example, aplurality of light emitting elements 602 that emit light based on thereference current from one reference current source 902 are sub-groupedinto four sub-groups in accordance with the positions in the mainscanning direction. Note that the light emitting elements 602 includedin one sub-group form spots continuously in the main scanning direction.Then, one light amount correction value B is associated with onesub-group.

A spot correction value C is a value which is intended to correct alight amount difference attributed to the expansion of a spot made by alight emitting element 602 in the main scanning direction, and whichindicates the amount of displacement of the spot (hereinafter, a spotdisplacement amount) from a reference value. A spot correction value Cis set for each light emitting element 602 that makes an expanded spot.Note that a light emitting element 602 for which no spot correctionvalue C is set is construed to have a spot correction value C of 0.While the details will be described later, in a case where a spot madeby a light emitting element 602 is expanded in the main scanningdirection, the influence thereof varies depending on the tones.Specifically, in the case of a high tone, the density increases as aspot expands in the main scanning direction. Therefore, in a case wherea spot for forming a portion with a high tone value is expanded in themain scanning direction, the light amount is reduced. On the other hand,in the case of a low tone, the density decreases as a spot expands inthe main scanning direction. Therefore, in a case where a spot forforming a portion with a low tone value is expanded in the main scanningdirection, the light amount is increased. Note that the absolute valueof the amount of increase or decrease in the light amount increases withan increase in a spot displacement amount.

The correction information that includes the light amount correctionvalues A, light amount correction values B, and spot correction values Cis stored into the storage unit 810 before shipment. Furthermore, theCPU 811 can update the correction information stored in the storage unit810 by obtaining the light amount correction values A, light amountcorrection values B, and spot correction values C using a methoddescribed later.

The image data that has undergone the dithering processing in the imagedata generation unit 801 is input to a tone determination unit 1105 andan image correction unit 1109. As stated earlier, this image dataindicates whether to cause each light emitting element 602 to emit lightwhen exposing each line of the photosensitive member 102 in the mainscanning direction to light.

The tone determination unit 1105 determines the tone values of pixelsbased on the input image data, and notifies a tone-by-tone correctionunit 1106 of the same. The tone-by-tone correction unit 1106 includes atone-by-tone correction table. Note that the correction table isincluded in the correction information. The correction table is a tablethat indicates a reference light amount correction value on atone-by-tone basis. Note that a reference light amount correction valuehaving a positive value indicates that the light amount is to beincreased, whereas a reference light amount correction value having anegative value indicates that the light amount is to be reduced. Asstated earlier, in a case where a spot is expanded in the main scanningdirection, the influence thereof varies depending on the tones. Forexample, assume that the tones are categorized into three types, namelya low tone, an intermediate tone, and a high tone, with use of a firstthreshold and a second threshold. Note that the first threshold islarger than the second threshold, a tone value larger than the firstthreshold represents a high tone, a tone value smaller than the secondthreshold represents a low tone, and a tone value larger than or equalto the second threshold and smaller than or equal to the first thresholdrepresents an intermediate tone. A reference light amount correctionvalue indicated by the correction table has a positive value for a lowtone, has a negative value for a high tone, and is 0 for an intermediatetone. In other words, a negative reference light amount correction valueis 0 for a low tone and an intermediate tone, and a positive referencelight amount correction value is 0 for a high tone and an intermediatetone.

The tone-by-tone correction unit 1106 corrects the reference lightamount correction value of the tone of a pixel notified by the tonedetermination unit 1105 based on the spot displacement amount indicatedby the spot correction value C of the light emitting element 602 thatforms a dot composing this pixel, thereby obtaining a light amountcorrection value D of this dot. As one example, the tone-by-tonecorrection unit 1106 holds coefficient information indicating acorrespondence relationship between spot displacement amounts andcoefficients, and obtains the light amount correction value D bymultiplying the reference light amount correction value of the tonenotified by the tone determination unit 1105 by a coefficientcorresponding to the spot displacement amount. Note that the lightamount correction value D of a dot formed by a light emitting element602 for which no spot correction value C has been set, that is to say, alight emitting element 602 with a spot correction value C of 0, isalways 0. The tone-by-tone correction unit 1106 outputs data indicatingthe light amount correction values D of the respective dots that composethe image to the image correction unit 1109.

Based on the light amount correction values A and the light amountcorrection values B, a calculation unit 1107 obtains light amountcorrection values E of the respective light emitting elements 602 thatare placed at different positions in the main scanning direction. Thelight amount correction value E of a light emitting element 602 is a sumof the light amount correction value A of the group to which this lightemitting element 602 belongs, and the light amount correction value B ofthe sub-group to which this light emitting element 602 belongs. Whilethe details will be described later, the value of the sum of the lightamount correction value A of the group to which a light emitting element602 belongs, and the light amount correction value B of the sub-group towhich this light emitting element 602 belongs, is 0 or a negative value,and does not become a positive value. That is to say, this value of thesum is a value indicating that the light amount is to be maintained asis or reduced, and does not become a value indicating that the lightamount is to be increased. The light amount correction values E are alsocorrection values for the light amounts of the respective dots on oneline in the main scanning direction, which are formed by the lightemitting elements 602 placed at different positions in the main scanningdirection. The calculation unit 1107 outputs the light amount correctionvalues E of the respective dots on one line in the main scanningdirection to the image correction unit 1109.

The image correction unit 1109 divides the data indicating the lightamount correction values D of the respective dots that compose the imageinto first data indicating the light amount correction values D of dotsfor increasing the light amount, and second data indicating the lightamount correction values D of dots for reducing the light amount.Furthermore, based on the light amount correction values E of therespective dots on one line in the main scanning direction, the imagecorrection unit 1109 generates third data indicating the light amountcorrection values E of the respective dots that compose the image. Then,the image correction unit 1109 adds the absolute values of the lightamount correction values D in the second data and the absolute values ofthe light amount correction values E of the same dots in the third data,thereby generating fourth data indicating the total light amountcorrection values of the respective dots that compose the image. Thetotal light amount correction values of the respective dots indicated bythe fourth data indicate that the amount of reduction in the lightamount is 0 or more, and will be hereinafter referred to as subtractiondata. On the other hand, the light amount correction values D of therespective dots indicated by the first data indicate that the amount ofincrease in the light amount is 0 or more, and will be hereinafterreferred to as addition data. The image correction unit 1109 correctsthe image data based on the subtraction data and the addition data. Inthe present embodiment, the image correction unit 1109 performs imagecorrection in units of partial images of a predetermined size, which areparts of the image to be formed. In the present example, it is assumedthat the size of a partial image is 10×10 pixels (a total of 100pixels). FIG. 11A shows one example of a partial image, which is aportion corresponding to 10×10 pixels in the image formed by thepre-correction image data. In FIG. 11A, one cell represents one pixel.Note that hatched pixels denote pixels to which toner is to be applied,whereas white pixels denote pixels to which toner is not to be applied.

In the present embodiment, it is assumed that one pixel is formed of 10continuous dots, both in the main scanning direction and in the subscanning direction. FIG. 11B shows an example of image data for formingone pixel of FIG. 11A. Hereinafter, the 10 light emitting elements 602in the main scanning direction that form one pixel of FIG. 11B will bereferred to as light emitting elements #1 to #10. In FIG. 11B, theK^(th) cell from the left (where K is an integer from 1 to 10) denotes adot (spot) made by the light emitting element #K. Specifically, ahatched cell denotes an exposure dot, or indicates that the lightemitting element #K is to emit light, whereas a white cell denotes anon-exposure dot, or indicates that the light emitting element #K is notto emit light. Note that the up-down direction in FIG. 11B correspondsto positions in the sub scanning direction. According to theillustration, for example, the light emitting element #1 emits light atthe first to fourth positions, the seventh position, and the eighthposition among the positions of formation of 10 dots in the sub scanningdirection. FIG. 11B can be deemed to show whether each of the 10×10 dotsthat form one pixel is an exposure dot or a non-exposure dot.

Below, the dots (spots) shown in FIG. 11B to FIG. 11D are identifiedusing the numbers in the main scanning direction and the sub scanningdirection. Regarding the numbers in the main scanning direction, theleftmost dot is denoted by 1, and the rightmost dot is denoted by 10.Regarding the numbers in the sub scanning direction, the uppermost dotis denoted by 1, and the lowermost dot is denoted by 10. Furthermore,for example, the dot that is positioned second in the main scanningdirection and the third in the sub scanning direction is denoted by (2,3).

The image correction unit 1109 includes a threshold matrix table forsubtraction, and a threshold matrix table for addition. The thresholdmatrix tables are tables indicating thresholds for 10×10 pixels, namely100×100 dots targeted for image correction. The image correction unit1109 compares the absolute value of the total light amount correctionvalue of a dot corresponding to a partial image among the dots in theimage indicated by the subtraction data, with the correspondingthreshold for the dot indicated by the threshold matrix table forsubtraction. Then, the image correction unit 1109 determines a dot forwhich the absolute value of the total light amount correction valueexceeds the threshold in the threshold matrix table for subtraction as afirst change dot.

FIG. 11C shows an example of the result of determination that has beenmade using the threshold matrix table for subtraction with respect to aportion equivalent to one pixel corresponding to FIG. 11B. In FIG. 11C,the dots at the positions (4, 2), (7, 5), (2, 8), and (8, 10) aredetermined as the first change dots. In a case where a first change dotis an exposure dot, the image correction unit 1109 changes this firstchange dot to a non-exposure dot. On the other hand, in a case where afirst change dot is a non-exposure dot, the image correction unit 1109leaves this first change dot as the non-exposure dot. Therefore, theimage correction unit 1109 corrects the image data of FIG. 11B as shownin FIG. 11D. The dot at the position (4, 2) in FIG. 11B is originally anon-exposure dot, and thus still remains as the non-exposure dot afterthe correction. On the other hand, the dots at the positions (7, 5), (2,8), and (8, 10) have been corrected from exposure dots to non-exposuredots.

The image correction unit 1109 similarly determines second change dotsusing the addition data and the threshold matrix table for addition. Ina case where a second change dot is a non-exposure dot, the imagecorrection unit 1109 changes this second change dot to an exposure dot.On the other hand, in a case where a second change dot is an exposuredot, the image correction unit 1109 leaves this second change dot as theexposure dot. In a case where the same dot has been selected both as afirst change dot and as a second change dot, the image correction unit1109 does not change the exposed/non-exposed state of this dot, andleaves this dot in a state indicated by the original image data. Notethat tables with high spatial-frequency characteristics, which are usedin a commonly-known blue noise mask method, can be used as the thresholdmatrix tables.

Regarding the threshold matrix tables (100×100 dots in the presentexample), the same tables are repeatedly used in the main scanningdirection and the sub scanning direction. However, as the subtractiondata and the addition data to be compared correspond to the entirety ofthe image and are not something that are repeated in a certain cycle,the occurrence of image defects on the borders of processing can besuppressed. Note that the thresholds of the threshold matrix tables areset so that the interval between the first change dots and the intervalbetween the second change dots are not even. The occurrence of moire canbe prevented by making the interval between the first change dots andthe interval between the second change dots uneven.

Furthermore, the light amount can be corrected with high precision byusing a correction resolution that is sufficiently high relative to thepixel size of the original image data. In addition, by correcting thelight amount before the chip data conversion unit 803 performs thedivision into pieces of image data for the respective light emittingchips 400, the occurrence of image defects on the borders of the lightemitting chips 400 can be suppressed compared to a configuration thatcorrects the light amount after the division.

As described above, according to the present embodiment,exposure/non-exposure of dots obtained by dividing one pixel shown inimage data is corrected in units of partial images of a predeterminedarea (10×10 pixels in the present example). With this configuration,simple and high-precision correction can be performed compared tocorrection of the light amount using complicated analog circuits.Furthermore, the occurrence of image defects can be prevented bycorrecting the light amount based on the light amount correction valuesof the respective light emitting elements 602 that continuously formspots in the main scanning direction, that is to say, the light amountcorrection values of the respective positions that are continuous in themain scanning direction. In addition, the light amount can be correctedwith high precision by correcting the light amount in units of dotsobtained by dividing one pixel.

In the present embodiment, the fluctuations in the light amounts of thelight emitting elements 602 in the main scanning direction are correctedin two steps. First, as correction in a first step, the fluctuations inthe light amount between the light emitting chips 400 are corrected byadjusting the digital values set in the D/As 901 of the light emittingchips 400. In order to decide on the digital value to be set in the D/A901 of each light emitting chip 400, all of the light emitting elements602 inside the light emitting chip 400 are caused to emit light, and thelight amount value of each light emitting element 602 in this lightemitting chip 400 is measured. Then, the digital value to be set in theD/A 901 is decided on so that the smallest one of the light amounts ofthe light emitting elements 602 inside the light emitting chip 400 isused as a target light amount. In this way, the light amounts of all ofthe light emitting elements 602 inside the light emitting chip 400 areequal to or larger than the target light amount. Therefore, as statedearlier, the light amount correction value E of each light emittingelement is a value indicating that the light amount is to be maintainedas is or reduced. By correcting the light amount in the first step usingthe digital values set in the D/As 901, the amount of correction in thelight amount correction unit 802 can be reduced, and as a result,deterioration in the image quality caused by correction of image datacan be suppressed.

Correction in a second step is correction of the fluctuations in thelight amount inside a light emitting chip 400, and is executed by thelight amount correction unit 802 correcting image data in theabove-described manner. The light amount correction values A, lightamount correction values B, and spot correction values C used by thelight amount correction unit 802, as well as the aforementioned digitalvalues input to the D/As 901 of the respective light emitting chips 400,are generated based on the result of measurement during an assembly andadjustment process for the exposure head 106, and stored into thestorage unit 810 as the correction information. The CPU 811 reads outthe correction information, sets the light amount correction values A,light amount correction values B, and spot correction values C in thelight amount correction unit 802, and further sets digital values in theD/As 901 of the respective light emitting chips 400.

Note that a spot correction value C indicates the amount of displacementof a spot from a reference value (a spot displacement amount). In orderto measure the spot correction values C, each of the light emittingelements 602 is caused to emit light individually. Then, the spot sizesare measured by reading the spots using a CCD camera, the amounts ofchange from the reference value are measured, and the relationships withthe occurrence positions, that is to say, the light emitting elements602 are used as the spot correction values C.

Note that the correction information can be generated inside the imageforming apparatus. FIG. 12 shows a light amount correction chart, whichis a measurement image formed on a sheet in order to generate thecorrection information. The light amount correction chart is formed bythe image forming apparatus in response to an instruction issued by auser to execute the adjustment of unevenness in the light amount. Theuser causes the reading unit 100 to read the sheet on which the lightamount correction chart has been formed. Consequently, the CPU 811obtains chart data, which is the result of reading by the reading unit100. The chart data is data that indicates a density distribution in themain scanning direction for each of a plurality of tone images 2101 to2106.

The light amount correction chart includes the plurality of tone images2101 to 2106 that are in the shape of a strip along the main scanningdirection, and reference marks 2111-1 to 2119-19 and 2112-1 to 2112-19that are placed above and below the tone images 2101 to 2106. Eachreference mark is a marker image for specifying the position of eachlight emitting chip 400, and is formed by emission of light by the lightemitting elements 602 located on the edge of each light emitting chip400 in the main scanning direction. For example, the reference mark2111-2 is formed by emission of light by four light emitting elements602 on the right edge of the light emitting chip 400-2, and four lightemitting elements 602 on the left edge of the light emitting chip 400-3.The CPU 811 determines each reference mark based on the chart data.Then, with respect to each of the tone images 2101 to 2106, the CPU 811determines the regions that have been formed respectively by the lightemitting chips 400-1 to 400-20 with use of the lines connecting thereference marks 2111-p (where p is an integer from 1 to 19) and thereference marks 2112-p. For example, it is determined that a region B1in FIG. 12 has been formed by the light emitting chip 400-2. Note, it isdetermined that the left edge of FIG. 12 has been formed by the lightemitting chip 400-1, and the right edge of FIG. 12 has been formed bythe light emitting chip 400-20.

The set of tone images 2101 and 2102 is formed from pieces of image datahaving the same tone value. However, in forming the tone image 2102, theCPU 811 reduces the digital value to be set in the D/A 901 of each lightemitting chip 400, by a predetermined rate, compared to the digitalvalue that was set in the D/A 901 of each light emitting chip 400 informing the tone image 2101. This makes the density of the tone image2102 lower than the density of the tone image 2101. The same goes forthe set of tone images 2103 and 2104, and the set of tone images 2105and 2106. Note that the tone values indicated by pieces of image datathat are respectively used to form the set of tone images 2101 and 2102,the set of tone images 2103 and 2104, and the set of tone images 2105and 2106 differ from one another. Specifically, the pieces of image datafor forming the set of tone images 2101 and 2102 are set to indicate thelargest tone value, and the pieces of image data for forming the set oftone images 2105 and 2106 are set to indicate the smallest tone value.Note that the density of the tone image 2102 is higher than the densityof the tone image 2103, and the density of the tone image 2104 is higherthan the density of the tone image 2105.

Next, a method of converting the chart data read by the reading unit 100into light amount data will be described using FIG. 13 . The CPU 811obtains average densities 2101_D to 2106_D of the tone images 2101 to2016, respectively, by averaging the densities read at respectivepositions in the main scanning direction. FIG. 13 shows a relationshipbetween the digital values that were set in the D/As 901 in forming thetone images 2101 to 2106, respectively, and the average densities 2101_Dto 2106_D. With respect to the set of tone images 2101 and 2102, the CPU811 obtains the amount of change k1 in the digital value relative to thechange in density. Specifically, the CPU 811 obtains the amount ofchange k1 by dividing the difference between the digital values thatwere set in the D/As 901 in forming the tone images 2101 and 2102 by thedifference between the average density 2101_D and the average density2102_D. Similarly, the CPU 811 obtains the amount of change k2 for theset of tone images 2103 and 2104, and the amount of change k3 for theset of tone images 2104 and 2105.

The CPU 811 obtains a light amount distribution of the tone image 2101in the main scanning direction by multiplying the densities atrespective positions of the tone image 2101 in the main scanningdirection, which are determined based on the chart data, by theinclination k1. Similarly, the CPU 811 obtains the light amountdistribution in the main scanning direction with respect to the toneimage 2103 and the tone image 2105 as well. Note that it is alsopermissible to adopt a configuration in which the light amountdistribution is obtained by changing the tones of the pieces of imagedata for forming the tone images 2101 and 2102, instead of changing thedigital values set in the D/As 901.

In the present embodiment, the CPU 811 obtains the digital values set inthe D/As 901, the light amount correction values A. and the light amountcorrection values B based on the light amount distribution of the toneimage 2103, which is an image of an intermediate-density region.Furthermore, the CPU 811 obtains the spot correction values C based onthe light amount distributions of the tone image 2101 and the tone image2105, which are a high-density region and a low-density region,respectively. Below, a method of obtaining the digital values set in theD/As 901, the light amount correction values A, and the light amountcorrection values B will be described using FIG. 14 .

FIG. 14 shows the light amount distribution of the tone image 2103. Notethat FIG. 14 only shows a portion corresponding to one light emittingchip 400. As has been described using FIG. 12 , which portion of thetone image 2103 was formed by which light emitting chip 400 can bedetermined using the reference marks. The CPU 811 decides on the digitalvalue to be set in the D/As 901 so that the smallest light amount inFIG. 14 is used as a target light amount T (a target value).Specifically, the CPU 811 decides on the digital value to be set in theD/As 901 by increasing the digital value that was set in the D/As 901 informing the tone image 2103 by a digital value corresponding to a valueobtained by subtracting the smallest light amount in FIG. 14 from thetarget light amount T.

A light amount 2301 in FIG. 14 is a light amount at a predeterminedposition inside a range where dots are formed by the plurality of lightemitting elements 602 inside the group corresponding to the referencecurrent source 902-1. Similarly, light amounts 2302 to 2305 arerespectively light amounts at predetermined positions inside the rangeswhere dots are formed by the plurality of light emitting elements 602inside the groups corresponding to the reference current sources 902-2to 902-5. For example, the CPU 811 uses the difference between the lightamount 2301 and the smallest light amount in FIG. 14 as the light amountcorrection value A associated with the reference current source 902-1.Note that as stated earlier, due to the digital value set in the D/As901, the smallest light amount in FIG. 14 is used as the target lightamount T. The CPU 811 similarly obtains the light amount correctionvalues A associated with the reference current sources 902-2 to 902-5 aswell.

Furthermore, for example, the CPU 811 determines the light amounts atfour positions from within the range where dots are formed by theplurality of light emitting elements 602 inside the group correspondingto the reference current source 902-1. Note that the positions at whichthe light amounts are determined are each selected from within the rangewhere dots are formed by the plurality of light emitting elements 602 inone sub-group. The CPU 811 uses the differences between the fourdetermined light amounts and the light amount 2301 as the light amountcorrection values B that are associated with the respective sub-groupsunder the reference current source 902-1. The CPU 811 similarly obtainsthe light amount correction values B associated with the referencecurrent sources 902-2 to 902-5 as well.

As described above, the entirety of the light emitting elements 602inside a group is corrected using a light amount correction value A,which is based on a reference current source 902, and the fluctuationsin the light amounts of the light emitting elements 602 inside the groupare corrected using light amount correction values B. With thisconfiguration, light amount correction values B can be represented usinga small number of bits, and the data amount of the correctioninformation can be reduced. As one example, a light amount correctionvalue A can be represented using four bits, and light amount correctionvalues B indicating the fluctuations in the light amounts inside agroup, that is to say, residual components can be represented using twobits.

Next, a method of obtaining the spot correction values C will bedescribed. FIG. 15A shows the light amount distributions of the toneimages 2101, 2103, and 2105. Note that FIG. 15A shows normalized lightamounts of the respective tone images 2101, 2103, and 2105. That is tosay, with regard to the tone image 2101, the values obtained by dividingthe light amounts at respective positions of the tone image 2101 in themain scanning direction by the average light amount of the tone image2101 are used as the values along the vertical axis of FIG. 15A. Thesame goes for the tone images 2103 and 2105. As a result ofstandardization, the light amounts of the tone images 2101, 2103, and2105 have similar values at most of the positions in the main scanningdirection.

However, if the spots made by the light emitting elements 602 locallychange due to manufacturing variations of the exposure head 106, thelight amounts of the tone images 2101, 2103, and 2105 start to vary.Specifically, in the case of the tone image 2105, which represents a lowtone, a sufficient light emission intensity is not obtained and thedensity decreases if the spots are locally increased. That is to say,when converted into the light amounts, the light amounts decrease asindicated by reference sign 2307 of FIG. 15A. On the other hand, in thecase of the tone image 2101, which represents a high tone, decreasing agap between neighboring pixels causes an increase in the density. Thatis to say, when converted into the light amounts, the light amountsincrease as indicated by reference sign 2306 of FIG. 15A. Note thatreference sign 2308 of FIG. 15A denotes the light amount of the toneimage 2103, which represents an intermediate tone.

In FIG. 15B, a solid line denotes the density characteristic for a casewhere the spots do not fluctuate, whereas a dotted line denotes thedensity characteristic for a case where the spots have become largerthan a standard. As shown in FIG. 15B, when the spots have become largerthan the standard, the density increases in a high-tone region, and thedensity decreases in a low-tone region. Note that the influence in anintermediate-tone region is small.

The CPU 811 obtains a peak value difference, which is a differencebetween a peak value of the normalized light amounts of the tone image2101 (reference sign 2306 of FIG. 15A) and a peak value of thenormalized light amounts of the tone image 2105 (reference sign 2307 ofFIG. 15A). Determination information indicating a relationship betweenpeak value differences and the spot displacement amounts that have beenobtained experimentally, and is stored in the image forming apparatus inadvance. By using the determination information based on the obtainedpeak value difference, the CPU 811 obtains the spot correction values Cassociated with the light emitting elements 602 corresponding to thepositions in the main scanning direction at which the normalized lightamounts have fluctuated.

Note that the specific values that have been used in the description ofthe present embodiment are examples, and the present invention is notlimited to using these specific values.

As described above, in the present embodiment, the fluctuations in thelight amounts of the respective light emitting elements 602 in the mainscanning direction are corrected in two steps. First, the imagecontroller 800 corrects the light amount difference between lightemitting chips 400 using the digital values to be set in the D/As 901inside the light emitting chips 400. Then, the image controller 800corrects the fluctuations in the light amounts of the respective lightemitting elements 602 inside the light emitting chips 400 by correctingimage data. By correcting the light amount difference between lightemitting chips 400 using the digital values to be set in the D/As 901inside the light emitting chips 400, the amount of correction of theimage data can be reduced, and deterioration in the image quality causedby the correction of the image data can be suppressed. Also, bycorrecting the light amount difference between light emitting elements602 inside the light emitting chips 400 by way of correction o the imagedata, simple and high-precision correction can be performed compared toa configuration provided with a correction circuit for correcting thecurrents flowing through the light emitting elements 602 on anindividual basis. That is to say, with the configuration of the presentembodiment, density unevenness can be suppressed without increasing thecircuit scale compared to a case where a correction circuit forcorrecting the currents flowing through the respective light emittingelements 602 inside the light emitting chips 400 is provided in thechips.

Furthermore, correction of the image data is performed, in units ofpartial images, by changing exposure dots and non-exposure dots usingthe threshold matrices having the same size as these partial images. Thesame threshold matrices are used repeatedly with respect to each of thepartial images that compose an image. However, as the light amountcorrection values to be compared with the threshold matrices correspondto the entirety of the image and are irrelevant to the size of thethreshold matrices, the occurrence of image defects on the borders ofthe partial images can be suppressed. In addition, as exposuredots/non-exposure dots are changed in units of multiple dots thatcompose one pixel, light amount correction can be performed with highprecision.

OTHER EMBODIMENTS

Embodiment(s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2022-048422, filed Mar. 24, 2022, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image forming apparatus, comprising: aphotosensitive member that is driven to rotate; an exposure headincluding a first light emitting chip and a second light emitting chipthat is placed at a position different from a position of the firstlight emitting chip in a direction along a rotation axis of thephotosensitive member, the first light emitting chip including aplurality of first light emitting elements that are placed at differentpositions in the direction along the rotation axis of the photosensitivemember, a first digital-analog converter that outputs a voltagecorresponding to a setting value as a digital value, and a first circuitunit that supplies a current to the plurality of first light emittingelements based on the voltage output from the first digital-analogconverter, the second light emitting chip including a plurality ofsecond light emitting elements that are placed at different positions inthe direction along the rotation axis of the photosensitive member, asecond digital-analog converter that outputs a voltage corresponding toa setting value as the digital value, and a second circuit unit thatsupplies a current to the plurality of second light emitting elementsbased on the voltage output from the second digital-analog converter;and at least one processor configured to set a value that has beendecided on so that one light emitting element included among theplurality of first light emitting elements emits light of apredetermined amount as the setting value in the first light emittingchip, and set a value that has been decided on so that one lightemitting element included among the plurality of second light emittingelements emits light of the predetermined amount as the setting value inthe second light emitting chip, generate image data pieces for causingthe plurality of light emitting elements to emit light, and correctimage data pieces that respectively correspond to the plurality of firstlight emitting elements based on first correction data for correctingamounts of light respectively emitted by the plurality of first lightemitting elements, and correct image data pieces that respectivelycorrespond to the plurality of second light emitting elements based onsecond correction data for correcting amounts of light respectivelyemitted by the plurality of second light emitting elements, wherein thefirst circuit unit supplies a current to each of the plurality of firstlight emitting elements based on the corrected image data pieces, andthe second circuit unit supplies a current to each of the plurality ofsecond light emitting elements based on the corrected image data pieces.2. The image forming apparatus according to claim 1, wherein the imagedata pieces are data pieces indicating whether to cause the respectivelight emitting elements to emit light, and the first correction data andthe second correction data are data for changing an image data pieceindicating that a light emitting element is to emit light, to an imagedata piece indicating that the light emitting element is not to emitlight.
 3. The image forming apparatus according to claim 2, wherein theone light emitting element included among the plurality of first lightemitting elements is a light emitting element with the lowest lightamount among the plurality of first light emitting elements, and the onelight emitting element included among the plurality of second lightemitting elements is a light emitting element with the lowest lightamount among the plurality of second light emitting elements.
 4. Theimage forming apparatus according to claim 1, wherein the plurality offirst light emitting elements include light emitting elements of a firstgroup and light emitting elements of a second group, the first circuitunit includes a first current source that supplies a current to thelight emitting elements of the first group, and a second current sourcethat supplies a current to the light emitting elements of the secondgroup, and the first correction data includes data corresponding to thefirst current source and data corresponding to the second currentsource.
 5. The image forming apparatus according to claim 4, wherein thedata corresponding to the first current source is a difference betweenan amount of light emitted by one light emitting element included amongthe light emitting elements of the first group and the predeterminedamount, and the data corresponding to the second current source is adifference between an amount of light emitted by one light emittingelement included among the light emitting elements of the second groupand the predetermined amount.
 6. The image forming apparatus accordingto claim 4, wherein the light emitting elements of the first groupinclude light emitting elements of a first sub-group and light emittingelements of a second sub-group, the light emitting elements of thesecond group include light emitting elements of a third sub-group andlight emitting elements of a fourth sub-group, and the first correctiondata includes data pieces that respectively correspond to the lightemitting elements of the first sub-group, the light emitting elements ofthe second sub-group, the light emitting elements of the thirdsub-group, and the light emitting elements of the fourth sub-group. 7.The image forming apparatus according to claim 6, wherein the data piececorresponding to the light emitting elements of the first sub-group is adifference between an amount of light emitted by one light emittingelement included among the light emitting elements of the firstsub-group and the predetermined amount, the data piece corresponding tothe light emitting elements of the second sub-group is a differencebetween an amount of light emitted by one light emitting elementincluded among the light emitting elements of the second sub-group andthe predetermined amount, the data piece corresponding to the lightemitting elements of the third sub-group is a difference between anamount of light emitted by one light emitting element included among thelight emitting elements of the third sub-group and the predeterminedamount, and the data piece corresponding to the light emitting elementsof the fourth sub-group is a difference between an amount of lightemitted by one light emitting element included among the light emittingelements of the fourth sub-group and the predetermined amount.
 8. Theimage forming apparatus according to claim 1, wherein the firstcorrection data includes data indicating amounts of displacement ofsizes of light spots on a surface of the photosensitive member from areference value, the light spots being respectively associated with theplurality of first light emitting elements.
 9. The image formingapparatus according to claim 1, further comprising: an image formingunit that forms an image on a sheet; and a reading unit that reads theimage on the sheet, wherein the at least one processor causes the imageforming unit to form a chart image including an image of a first toneand an image of a second tone on the sheet, and generates the correctiondata pieces based on a reading result of the chart image on the sheet bythe reading unit.
 10. The image forming apparatus according to claim 1,wherein the exposure head includes a storage unit that stores a valuecorresponding to the first digital-analog converter and a valuecorresponding to the second digital-analog converter, and the at leastone processor reads out the value corresponding to the firstdigital-analog converter from the storage unit and sets the value as thesetting value in the first digital-analog converter, and reads out thevalue corresponding to the second digital-analog converter from thestorage unit and sets the value as the setting value in the seconddigital-analog converter.
 11. The image forming apparatus according toclaim 1, wherein the plurality of first light emitting elements and theplurality of second light emitting elements are organic ELs.